1. Field of the Invention
The present invention relates to an image reader comprising an optical system for varying the magnification of an image.
2. Description of Prior Art
In a conventional image reader comprising an optical system for varying the magnification of an image, a lens is disposed on the optical path between a document and an image sensor, and the image reduced by the lens is formed on the image sensor. The magnification of reading the image can be changed optically by moving the position of the lens. When the magnification is changed, the lens and the image sensor are moved to a position where the image formed on the image sensor is focused. The moving amounts of the lens and the image sensor can be calculated according to predetermined equations. The lens and the image sensor are moved by pulse motors, respectively.
Upon reading the image, the optical system comprising the lens and the image sensor must be adjusted to be in the infocus condition. Therefore, when the power is supplied to the image reader, the optical system must be adjusted to be in the infocus condition by changing the distance between the lens and the image sensor.
Furthermore, on the focus adjustment, the focus adjustment may not be performed depending on the initial position of the optical system. For example, when the optical system is positioned at a stopper, the optical system can not be moved and the distance between the lens and the image sensor can not be changed even though a motor for the focus adjustment is rotated. Normally, an illumination lamp provided for the focus adjustment is turned on for a relatively short time. However, when the focus adjustment can not be performed as described above, the illumination lamp may continue to be turned on, resulting in that there is a problem of safety such as increase of temperature of the image reader.